Everyone is acting under that assumption, though. And Carol isn’t disputing it.
It’s a poor assumption, and Rick should be the one pointing it out, although anyone could if they applied common sense. Carol’s admittance was to Rick, so he doesn’t have to convey that to others.
But even if Hershel could show that they were alive (making Hershel the Quincy of veterinarians), it is still relatively easy for Rick to turn this into a non-issue. They were dying, and suffering terribly. They had earlier indicated their demand that their bodies be burned to prevent the virus from spreading. When they are beginning to become unable to breath, Carol wants to go get help, but realizes that they are essentially gone. Seeing their bodies beginning to convulse, and seeing them in complete agony, she euthanizes them, and then burns their bodies in accordance with their wishes.
Two little white lies, and problem solved.
What would T’s problem be then? That she didn’t let them suffer? That she honored their wishes? The only potential issue would be that she didn’t fess up earlier. But, she’s a woman, and therefore emotional. She was so upset at what she had to do that she couldn’t bring herself to talk about it. She was scared that no-one would understand. There are plenty of excuses. Plus, she didn’t deny it when Rick confronted her.